A sintered NdFeB magnet was discovered in 1982 by Sagawa, the inventor of this invention, and other researchers. Sintered NdFeB magnets exhibit characteristics far better than those of conventional permanent magnets, and can be advantageously manufactured from neodymium (a kind of rare earth element), iron, and boron, which are relatively abundant and inexpensive as raw materials. Hence, sintered NdFeB magnets are used in a variety of products such as a voice coil motor used for a hard disk drive or other apparatus, a driving motor of a hybrid or electric car, a motor for a battery-assisted bicycle, an industrial motor, a generator used for wind power generation or other power generation, high-grade speakers and headphones, and a permanent magnetic resonance imaging system. Sintered NdFeB magnets used for those purposes require a high coercive force HcJ, a high maximum energy product (BH)max, and a high squareness ratio SQ. The squareness ratio SQ is defined as Hk/HcJ, where Hk is the absolute value of the magnetic field measured when the magnetization intensity is decreased by 10% from the maximum on the magnetization curve.
One known method for enhancing the coercive force of a sintered NdFeB magnet is a single alloy method, in which a portion of Nd atoms in a starting alloy is substituted with Dy and/or Tb (hereinafter, “Dy and/or Tb” will be referred to as “RH”). Another known method is a “binary alloy blending technique” in which a main phase alloy and a grain boundary phase alloy are independently prepared, and RH is densely added into the grain boundary phase alloy to increase the density of RH at the grain boundaries among the crystal grains in a sintered compact and the area around the grain boundaries. Further, a “grain boundary diffusion method” is also known in which a sintered body of a NdFeB magnet is prepared and then RH is diffused from the surface of the sintered body to the inside thereof through the grain boundaries so that the concentration of RH will increase only in the area near the grain boundaries of the sintered compact (Patent Document 1).